1. Statement of the Technical Field
The inventive arrangements relate generally to micro-fluidic devices and more particularly to structures and systems for preventing fluid backflow.
2. Description of the Related Art
Micro-fluidic systems have the potential to play an increasingly important role in many developing technology areas. For example, there has been an increasing interest in recent years in the use of fluid dielectrics for use in RF systems. Likewise, conductive fluids can have use in RF systems as well.
Another technological field where micro-fluidic systems are likely to play an increasingly important role is fuel cells. Fuel cells generate electricity and heat by electrochemically combining a gaseous fuel and an oxidant gas, via an ion-conducting electrolyte. The process produces waste water as a byproduct of the reaction. This waste water must be transported away from the reaction to be exhausted from the system by a fluid management sub-system.
Efforts are currently under way to create very small fuel cells, called microcells. It is anticipated that such microcells may eventually be adapted for use in many portable electronics applications. For example, such devices could be used for powering laptop computers and cell phones. Still, microcells present a number of design challenges that will need to be overcome before these devices can be practically implemented. For example, miniaturized electro-mechanical systems must be developed for controlling the fuel cell reaction, delivering fuel to the reactive components and disposing of water produced in the reaction. In this regard, innovations in fuel cell designs are beginning to look to silicon processing and other techniques from the fields of microelectronics and micro-systems engineering.
Glass ceramic substrates sintered at 500° C. to 1,100° C. are commonly referred to as low-temperature co-fired ceramics (LTCC). This class of materials has a number of advantages that makes it especially useful as substrates for RF systems. For example, low temperature 951 co-fire Green Tape™ from Dupont® is Au and Ag compatible, and it has a thermal coefficient of expansion (TCE) and relative strength that are suitable for many applications. The material is available in thicknesses ranging from 114 μm to 254 μm and is designed for use as an insulating layer in hybrid circuits, multi-chip modules, single chip packages, and ceramic printed wire boards, including RF circuit boards. Similar products are available from other manufacturers.
LTCC substrate systems commonly combine many thin layers of ceramic and conductors. The individual layers are typically formed from a ceramic/glass frit that can be held together with a binder and formed into a sheet. The sheet is usually delivered in a roll in an unfired or “green” state. Hence, the common reference to such material as “green tape”. Conductors can be screened onto the layers of tape to form RF circuit elements antenna elements and transmission lines. Two or more layers of the same type of tape are then fired in an oven.
Many of the same characteristics that make LTCC an excellent choice for fabrication of microelectronic circuits also suggest its value for use in microfluidic applications. LTCC is mechanically stable at temperatures from below freezing to over 250° C., has known resistance to chemical attack from a wide range of fluids, produces no warpage during compression, and has superior properties of absorption as compared to other types of material. These factors, plus LTCC's proven suitability for manufacturing miniaturized RF circuits, make it a natural choice for manufacturing microfluidic systems including, but not limited to, fluid systems used in microcells.
Many of the applications for fuel cells and other types of fluid systems can require fluid control systems, and more particularly an ability to prevent backflow of fluids. Accordingly, check-valves that allow fluid to flow in only one direction are often needed in such systems. Conventional approaches to such check-valves can be implemented in micro-fluidic LTCC devices as discrete components added to the LTCC after firing. However, discrete components are typically mounted on the surface of the device and can create a higher profile. They also can tend to be less robust.
In the semiconductor area, there has been some development of micro electromechanical systems (MEMS) that include check-valves. However, these devices tend to have long development times, are difficult to interface in the macro world, and require more mechanical interfaces. In contrast, LTCC systems can involve a considerably shorter development time and are showing promise in the fuel cell area. Accordingly, integrated LTCC fluid flow components are important for the future of micro-fluidic systems for fuel cells and other technologies.